HUD Documentation - Evaluations and RL Environments.

Sometimes you want to reach an environment over a standard wire protocol instead of the SDK. This example puts one in front of Chat: it serves a messages-style task as an A2A endpoint, so any A2A client - including another LLM using it as a tool - can hold a conversation with your environment. A Chat is HUD’s multi-turn conversation object, and it is protocol-agnostic (see Chat). The A2A layer is a reference server kept outside the SDK on purpose, so you copy and adapt it from cookbooks/a2a-chat.

The pieces

File	What it does
`server.py`	A2A server: one `Chat` (conversation) per A2A context, agent card, citations artifact
`client.py`	Minimal A2A client: send messages, print replies
`llm_client.py`	LLM-fronted client: an OpenAI model decides when to call the A2A agent as a tool
`chat_env.py`	Sample chat environment with `messages`-style tasks to serve

The environment

A chat-style task takes the running conversation as a messages parameter and yields it as the prompt:

chat_env.py

from mcp.types import PromptMessage
from hud.environment import Environment

env = Environment(name="chat")

@env.template()
async def chat_simple(messages: list[PromptMessage]):
    yield messages   # the conversation so far is the prompt
    yield 1.0

Run it

From cookbooks/a2a-chat (uv resolves the dependencies on first run):

# Terminal 1: serve the bundled chat task (spawns chat_env.py per turn)
uv run server.py

# Terminal 2: talk to it
uv run client.py            # plain client
uv run llm_client.py        # LLM-fronted client

The server publishes an agent card at /.well-known/agent-card.json and accepts A2A messages at the root endpoint. Each A2A context gets its own Chat, so concurrent conversations stay independent. You configure the server through environment variables:

Variable	What it controls
`HUD_MODEL`	The agent’s model, routed through the gateway (needs `HUD_API_KEY`).
`HUD_TASK` / `HUD_ENV`	Which task row to serve.
`HUD_SOURCE`	Spawns a different env source.
`HUD_ENV_URL`	Attaches each turn to an already-served control channel instead of spawning. For example, `hud serve chat_env.py` then `HUD_ENV_URL=tcp://127.0.0.1:8765`.

Start here

Guides

Reference

Advanced

Cookbooks

More

A2A chat

The pieces

The environment

Run it

See also

Chat

Integrations

​The pieces

​The environment

​Run it

​See also

Chat

Integrations

The pieces

The environment

Run it

See also